tank de-rust

[mickwinf]

Ok i know it has been covered before but it is doing my head in! I have de- rusted the interior of my spare tank using phosphoric acid as recommended by Ashimoto. It works very well leaves it completely clean metal. The issue i have is it rusts again overnight, and had the same problem with my old tank. So these are the methods i have used with no success. If i flush with clean water then with petrol, it rusts. If i let dry without flushing the phosphoric acid it leaves a black tar like substance which only cleans off using more acid. I also tried swilling some diesel around and still it rusts! I don't want to line it as most people on here are dubious about them so what do i do? when the tanks are new they dont flash rust so why does it happen?

[Laverda Dave]

Mick, I seem to remember (I think) Nurse Julie recommending a bit of diesel oil sloshed around the tank as it contains oil and therefore keeps it rust free. Normal petrol contains ethanol that will rust your tank as it attracts water and holds in suspension.

[Nurse Julie]

Rince out with de ionised water and then swish diesel around inside the tank to create an oily layer.

[McCabe-Thiele (Ted)]

I have used stuff like Trustran (a rust convertor) that is like a gun blueing liquid on small items. I used Evaporust on my tank then swilled it with 2T oil. Quite a few rust convertors chemically react with the rust to form a none rusting oxide protective layer rather than removing the iron oxide from the metal thus enabling it to go rusty again.

[K2-K6]

"when the tanks are new they dont flash rust so why does it happen?"

When steel is run through the roller mill to progressively bring it to it's sheet thickness it effectively ends "forged" with very high compression and, to a certain degree, refined inpenetrability as the final surface. This is generally pretty resilient to surface corrosion as it's just so compacted,  with any subsequent disturbance causing reduction of that finish whether starting with corrosion which once removed have partly destroyed that finish.  All subsequent treatments by removing rust can't reinstate that first finish status, at which point it will need some element of protection.  What that is I'm not sure in terms of effectiveness, but it can't be just left alone.

[McCabe-Thiele (Ted)]

"when the tanks are new they dont flash rust so why does it happen?"

When steel is run through the roller mill to progressively bring it to it's sheet thickness it effectively ends "forged" with very high compression and, to a certain degree, refined inpenetrability as the final surface. This is generally pretty resilient to surface corrosion as it's just so compacted,  with any subsequent disturbance causing reduction of that finish whether starting with corrosion which once removed have partly destroyed that finish.  All subsequent treatments by removing rust can't reinstate that first finish status, at which point it will need some element of protection.  What that is I'm not sure in terms of effectiveness, but it can't be just left alone.

I assumed they had a coating of some sort like yellow zinc!

[K2-K6]

I'm not aware that they were coated in any way. By comparison some of the inside of fuel caps are zinc plate with a clear and obvious difference to the tank internal.

[Athame57]

Rince out with de ionised water and then swish diesel around inside the tank to create an oily layer.

I'd better hurry up and de-rust my spare tank then.....at this rate diesel will be nearly extinct!

[Oz350Four]

I've used 2 stroke fuel mix as I didn't have diesel at the time and it worked a treat...

[kevski]

As Julie says, rinse with D,I water then rinse with diesel, as a former metal chemist i could go deeper but won't.

[mickwinf]

Thanks for replies, i have tried these methods, the only difference to Julies was i used tap water to flush not distilled. Has diesel in at the moment and has rusted again.

[K2-K6]

You could try a rinse with a bi carbonate of soda straight after a water wash as it should neutralise any remaining acid to help prevent flash rusting and see what you get.

Don't put the bi carbonate in straight after acid, rinse first then go in with bi carbonate wash.

[K2-K6]

Washing Soda crystals or household bleach are heading up the ph scale if you want to asses the affects of bringing it more alkaline with easily available chemicals.

Sodium hydroxide "drain cleaner" is further out on the scale too.

[Kelly E]

I use EvapoRust and rinse with diesel. The last thing I am going to put in a freshly derusted tank is water. When you dump out the diesel into a jug let the sludge settle and pour the diesel back into the tank for another round of rinsing. I rinse at least 4 or 5 times. Never have had any flash rust with this method. The diesel can be used over and over again. Just keep pouring off the clean diesel and dump the sludge.

[Nurse Julie]

I use EvapoRust and rinse with diesel. The last thing I am going to put in a freshly derusted tank is water. When you dump out the diesel into a jug let the sludge settle and pour the diesel back into the tank for another round of rinsing. I rinse at least 4 or 5 times. Never have had any flash rust with this method. The diesel can be used over and over again. Just keep pouring off the clean diesel and dump the sludge.

De Ionised water is just fine, it's very cheap, readily available and prevents flash rusting. Then swill round with diesel.